Active ingredient in tobacco and its moa
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Nicotine - Active Ingredient in Tobacco
What is Nicotine?
Nicotine is the primary active ingredient in tobacco (Nicotiana tabacum). Other alkaloids present in tobacco include nornicotine, nicotianine, and anabasine, but nicotine is the principal one responsible for pharmacological effects and addiction. It is a colorless, volatile liquid alkaloid (pKa = 8.5) that turns brown on exposure to air, and it is second only to alcohol as the most abused drug worldwide.
- Lippincott Pharmacology, p. 738
- Essentials of Forensic Medicine & Toxicology, 36th ed., p. 557
Mechanism of Action
1. Receptor Target - Nicotinic Acetylcholine Receptors (nAChRs)
Nicotine binds to nicotinic acetylcholine receptors (nAChRs), which are ligand-gated ion channels found throughout the CNS and peripheral nervous system. It mimics acetylcholine at these sites. The key receptor subtype for addiction is the α4β2 nAChR, found in the ventral tegmental area (VTA) of the brain.
2. Dose-Dependent Biphasic Action
This is a defining feature of nicotine:
| Dose | Effect |
|---|---|
| Low dose | Ganglionic stimulation via depolarization |
| High dose | Ganglionic blockade via receptor desensitization |
The ultimate physiological response is always a summation of stimulatory and inhibitory effects, making nicotine's effects complex and unpredictable. - Goodman & Gilman's Pharmacological Basis of Therapeutics
3. CNS and Reward Pathway (Addiction Mechanism)
Nicotine exerts its addictive effects through three converging mechanisms in the mesolimbic dopamine system:
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Direct dopamine release: Nicotine binds to α4β2 postsynaptic nAChRs on dopamine neurons in the VTA, directly triggering dopamine release in the nucleus accumbens (reward center).
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Glutamate-mediated dopamine release: Nicotine binds to α7 presynaptic nAChRs on glutamate (Glu) neurons in the VTA, stimulating glutamate release, which in turn drives further dopamine release in the nucleus accumbens.
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Disinhibition via GABA suppression: Nicotine desensitizes α4β2 receptors on GABA interneurons in the VTA. With GABA neurotransmission reduced, the inhibitory brake on dopamine neurons is lifted, causing even more dopamine to flood the nucleus accumbens.
- Stahl's Essential Psychopharmacology, p. 564-565
4. Peripheral Nervous System Effects
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Sympathetic ganglia stimulation + adrenal medulla discharge → release of catecholamines → increased heart rate and blood pressure
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Neuromuscular junction: Low doses stimulate; high doses cause paralysis via receptor desensitization
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Sensory receptor stimulation: Mechanoreceptors, chemoreceptors of the carotid/aortic bodies, pain receptors
-
Goodman & Gilman's
5. CNS Effects
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Low doses: mild analgesia, euphoria, arousal, relaxation, improved attention and learning
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High doses: tremors, convulsions, then CNS depression
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Respiratory stimulation at moderate doses (via carotid/aortic chemoreceptors); respiratory paralysis at toxic doses (via medullary depression)
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Also acts as an appetite suppressant
-
Lippincott Pharmacology, p. 738
Receptor Upregulation and Addiction Cycle
Chronic exposure causes the brain to upregulate α4β2 receptors as a compensatory response to prolonged desensitization. This backfires because:
- More receptors = amplified craving when they resensitize between cigarettes
- The smoker then smokes again to desensitize them and suppress craving
- This reinforcing cycle is the neurobiological basis of nicotine dependence
PET scan studies confirm that a single cigarette delivers enough nicotine for just long enough to desensitize most α4β2 receptors, with craving beginning at the first signs of receptor resensitization. - Stahl's Essential Psychopharmacology, p. 564
Summary Table
| Feature | Detail |
|---|---|
| Chemical class | Tertiary amine alkaloid |
| Primary receptor | nAChRs (especially α4β2) |
| Key action site | Ventral tegmental area (VTA) → nucleus accumbens |
| Neurotransmitter released | Dopamine (main), also glutamate, norepinephrine |
| Peripheral effect | Biphasic - stimulation then blockade of autonomic ganglia |
| Fatal dose | 50-100 mg pure nicotine; ~15-30 g crude tobacco |
| Addiction mediator | Dopamine release + receptor upregulation cycle |
Forensic note: Acute nicotine poisoning causes early parasympathetic symptoms (nausea, hypersalivation, miosis, bradycardia) followed by sympathetic stimulation and then CNS depression. Death results from respiratory failure. The specific antidote is mecamylamine (ganglionic blocker). - Essentials of Forensic Medicine & Toxicology, 36th ed.
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